When is the First Frost?

This year (2017) the first frost at Manilla came on the 11th of May, close to the middle date for it: the 13th of May. In just half of the years, the first frost comes between ANZAC Day (the 25th of April) and the 19th of May.

Graphical record of first frost dates
(See the notes below: “Observing Frosts in Manilla.”)

The date of first frost from year to year

The graph shows the dates of first frosts in the last nineteen years. One feature stands out: from a very early date of the 4th of April in 2008, the dates got later each year to a very late date of the 8th of June in 2014. Otherwise, the dates simply jumped around.

Graphical log of frostsThe date of first frost hardly relates at all to the number of frosts in a season. This graph, copied from an earlier post, shows how poorly they match. The earliest first frost, in 2008, was in a year with a normal number of frosts. In the least frosty year, 2013, the first frost did not come late.

The central date and the spread

To find the central value and the spread of a climate item like this calls for readings for a number of years called a “Normal Period”. (See note below on Climate Normals.) I chose the first eleven years of my readings (1999 to 2009) as my Normal Period. For this period I found these five order statistics:

Lowest (earliest) value: 4th April;
First Quartile value: 25th April (ANZAC Day);
Median (middle) value: 13th May;
Third Quartile value: 19th May;
Highest (latest) value: 24th May.

These five values divide the dates of first frost into four equal groups. For example, the first frost comes before ANZAC Day in one year out of four. This could confirm what Manilla gardeners know already!

Is the first frost getting later?

Talk of global warming leads us to expect the date of first frost to get later. By how much?
Dates on the graph after 2009 seem to be later in the season than during the Normal Period. As shown, a linear trend line fitted to the data points slopes steeply down towards later dates in later years. A curved trend line (a parabola) slopes down even more steeply. However, with so few data points, these trend lines are wild guesses, not to be relied on for forecasting future frosts.
Australian Bureau of Meteorology data for NSW from 1910 shows that daily minimum temperatures have been rising at 0.11° per decade.  (That is much faster than the rate for daily maximum temperatures, which is 0.07° per decade.) To work out how this might affect the date of first frost in Manilla, one needs to know that the daily minimum temperature in this season gets lower each day by 0.15°. One day of seasonal cooling will more than cover a decade of climate warming. The effect of global warming is to make the date of first frost only one day later in fourteen years. If the middle date of first frost was the 13th of May in the Normal Period, centred on 2004, the forecast middle date of first frost next year (2018) would be the 14th of May. This is shown by the flattest of the three trend lines on the graph.

Looking ahead, it seems unlikely that the date of first frost will get later by as much as a week within a lifetime.

[Update 12 August 2019:
This estimate of the median date of first frost, 13 May, is not changed by first frosts observed on 15 May 2018 and 12 May 2019.]


1. Observing Frost in Manilla

I have been reading daily maximum and minimum temperatures since March 1999 using an electronic thermometer protected by a Gill Screen. If others in Manilla also have been reading thermometers, it is likely that their record of frosts is different. There are two reasons.
Firstly, my thermometer is on high ground near the hospital. It is not that the temperature gets much colder due to higher altitude – quite the reverse. At night, the air near the ground gets cold as the ground loses heat to the sky. The cold air, being heavy, flows away into the hollows down by the river. The low ground gets colder frosts, and a thermometer there may show a first frost earlier than my thermometer does.
Secondly, a thermometer screen is more than a meter above the ground surface, while freezing water damages plants during a frost at the the ground surface. Usually, the minimum temperature read in a screen is warmer. I have chosen to record a frost whenever the screen temperature is below +2.2° Celsius. Others may not have used the same rule.

2. Climate Normals

The word “Normal” has a special sense in the study of climate.

The World Meteorological Organisation (WMO) defines Climatological standard normals as follows:

“Climatological standard normals: Averages of climatological data computed for the following consecutive periods of 30 years: 1 January 1901 to 31 December 1930, 1 January 1931 to 31 December 1960, etc. (Technical Regulations). WMO publishes the climatological standard normals which are computed by the WMO Members for their observing stations. (CLINO), WMO No.847. The latest version of this publication includes the standard normals for the period 1961-1990.”

Many countries, including Australia, have published normals for the period 1961-1990. With the observation of global warming, it has become clear that the 1961-1990 data is no longer current, yet it is some years before data for the next scheduled standard normal period, 1991-2020 will become available. There is some use of ” a “rolling” set of 30 year Normals updated every 10 years”.

The Australian Bureau of Meteorology (BoM) says:

“Climate normals
Statistics calculated over standard periods (commonly a 30 year interval) are often called climate normals, and are generally used as reference values for comparative purposes. The period is long enough to include the majority of typical year to year variations in the climate, but not so long that it is significantly influenced by longer-term changes in climate. In Australia, the current reference climate normal is generated over the 30-year period 1 January 1961 to 31 December 1990.”

I note that the Bureau of Meteorology does not insist on a 30 year interval for a normal.

The US National  Oceanic and Atmospheric Administration (NOAA) adds the following remarks:

“Many agencies, including NOAA’s Climate Prediction Center, develop their own averages and change base periods for specific applications and/or internal use. Similarly, numerous individuals calculate their own Climate Normals for a variety of reasons.”

My own use of Normals
After collecting some years of weather data, I found that I needed to set up my own climate normals. For Manilla, normals existed for rainfall only.  I had to make do with a period much shorter than 30 years, and commencing much later than 1961. For most variables, I used the decade commencing March 1999. I also made model curves that were more regular than raw 10-year averages.
For this frost analysis, I set up an 11-year normal period 1999-2009, so that the mid-year would not fall between the two years 2003/2004, but on the year 2004.

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